Cerium-Based Materials: Synthesis, Properties and Applications

By Ramesh Chandra and Ravi Tomar

Cerium is the most abundant metal of rare-earth elements. It can be used to make materials such as phosphors and alloys, that have applications in various applied fields (like electronics, magnetics and heterogeneous catalysis) and devices (like catalytic converters and gas mantles).

Cerium-Based Materials: Synthesis, Properties and Applications presents detailed knowledge about cerium materials. Starting with the history of cerium-based materials, it gives an introduction to the synthesis of chemicals like cerium oxides and composites. This is followed by information about characterization of cerium nanoparticles and industrial applications of cerium-based materials, with a focus on catalysis, biomedical engineering and pharmaceutical chemistry.

This book is an essential reference for researchers and chemical engineers who want a summary of cerium materials and its applications.

• Language: English
• Publisher: Bentham Science Publishers
• Release date: May 23, 2023
• ISBN: 9789815080087

Book preview

Introduction to Cerium and Cerium-based Materials

Shalu Atri¹, Shilpa¹, Ravi Tomar², *

¹ Department of Chemistry, Faculty of Science, SGT University, Gurugram, Haryana, India

² Department of Chemistry, University Center for Research and Development, Chandigarh University, Mohali, Punjab, India

Abstract

The redox behavior of cerium is responsible for its high technological importance since ceria serves as a potential material in assorted applications. Changes in the optical, electrical, magnetic, and catalytic behavior of ceria can be brought about by employing different methodologies and reaction conditions. The high thermal and structural stability of ceria offers its utilization as a host lattice for various doping schemes. The incorporation of dopant elements beautifies the lattice by generating oxygen vacancies and thereby creating various interesting properties. Aiming with the stabilization of ceria and ceria-based compounds in nano-dimensions also opens up various new possibilities to explore it further for numerous useful applications.

Keywords: Cerium, Cerium-based materials, Characterization, Nanomaterials, Synthesis.

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* Corresponding author Ravi Tomar: Department of Chemistry, University Center for Research and Development, Chandigarh University, Mohali, Punjab, India; E-mail: [e-mail]

Introduction

Cerium is a well known abundant rare-earth element. Its minerals such as silicate, carbonate, phosphate, and hydroxide can be utilized in various fields of applications, like phosphors, alloys, magnetics, catalysis, catalytic converters and gas mantles. In the last decades, China is the biggest producer of cerium. Even the price of cerium oxide is cheaper than lanthanum oxide. It can be extracted from its ores. Cerium exhibits variable oxidation states such as +4, +3, +2 and +1 oxidation states but among them, +4 is the most stable. In addition to this, Ce³+ compounds are also known to be stable such as Ce(NO3)3 and many more. Cerium oxide (ceria, CeO2) and ceria-based materials have been explored for various applications in academia and industries such as a catalyst, [1-20] a pharmaceutical

[21-23], in electrochemistry [24-28], as a sensor [29, 30], as an interlayer/sublayer/buffer layer [31, 32], and so on [33-37]. Cerium-based materials are most commonly used in heterogeneous catalysis.

Ceria is pale yellow powder which is obtained by the combustion of cerium, oxalate or hydroxide. Ceria received much attention due to its redox property, thermal stability, transport properties as well as oxygen storage capacity [38, 39]. It was first used industrially by Ford Motor Company in 1976 in catalytic converters as an oxygen storage component. On the other hand, ceria was considered to be an inert support. In past decades, ceria was used for the stabilization of catalytically active nanoparticles. After that it had been considered as a co-catalyst in catalytic reactions. But in recent times, experimentally it has been proved that it is a highly promising catalyst in numerous organic and inorganic reactions. Majorly it helps in the construction of three-way-catalyst (TWCs). Since 1950 till date, there are more than 26000 publications including the study related to CeO2 and ceria-based materials having a wide range of applications. The golden year of ceria materials in terms of publications was from 2014 to 2018. During these years, more than 2500 papers were published on ceria materials. Moreover available reports based on pristine and doped ceria show their applications in superoxide dismutase mimetic activity, hydroxyl radical scavenging, in the reduction of ischemic brain damage by disruption of the blood-brain barrier after ischemia, as a catalyst for intracellular drug delivery, as a support for stem cells in cultured vitro, peroxidase mimetic activity, oxidase mimetic activity, phosphate mimetic activity, nitric oxide radical scavenging and many more.

Ceria in its crystal structure is known to exist with fluorite lattice having space group Fm-3m (Fig. 1). In its structure, 8-coordination is exhibited by cerium ions and 4-coordination by oxygen ions. The availability of mixed oxidation state or redox properties makes it accountable for various applications. Usually, doping of ceria is carried out by substituting cerium ion with lower valence cations which introduces oxygen vacancies to maintain the overall charge neutrality in the lattice. Doping induced oxygen vacancies provide equal sites to migrate oxygen ions by hopping mechanism and are thereby responsible for high ionic conductivity of the lattice. Moreover, ceria is significantly capable of bringing out high degree of substitution which results in non-stoichiometric ceria. Consequently, non-stoichiometry of ceria provides highly disordered structures. Usually, in order to improve oxide ion conductivity of ceria, cerium ions are substituted by rare earth ions [40]. Reported literature includes that ceria can be doped with numerous metals as dopant which contribute to tuning the oxygen storage capacity, oxide-ion conductivity and redox behavior too. Dopant cations involve isovalent (Zr⁴+ and Sn⁴+), aliovalent (Y³+, Gd³+, Sm³+, La³+, Pr⁴+/³+ and Sr²+) and transition-metal ions (Ti⁴+, Cu²+, Fe³+ and Pd²+) to enhance oxide-ion conductivity [41-54].

Fig. (1))

Crystal structure of ceria along with representation of cerium ion (8-coordinated) and oxygen ion (4-coordinated).

Ceria-based materials

Scientists throughout the world gave two major directions for ceria-based materials that are of high interest in catalysis. From the industrial point of view, the first one is in car convertors, which can be fulfilled by increasing the surface area and thermal stability of ceria based materials. With this idea, solid solutions of transition or rare earth metals especially those containing zirconium have been prepared and found to be highly promising. These achievements facilitate their usages as closed coupled catalysts (CCC) that will be stable up to a temperature of 1000 ºC. One of such examples is ceria-zirconium based TWCs that play a role as environmental catalysts. Although continued debate is going on the necessities of phase homogeneity exhibited by ceria-based materials, other than zirconium, palladium-cerium based materials have a broad range of applications in organic synthesis for example in cross-coupling reaction, oxidation, hydrogenation, methane activation and many more.

Cerium-based as nanomaterials

Nanotechnology provides materials with a controlled shape and size (in nanodimensions). Although, low thermal stability of nanomaterials is a challenging task for researchers which forced them to understand the structural property relationship by computational studies. Such motivational approach has signified the dependence of soot oxidation reaction on the surface of catalysts and also examined the dependence of CO and propane oxidation reaction on the ceria crystal plane. A highly impressive study observed modulation of the ceria-based system with a metal support. Gold-supported ceria plays a crucial role in the oxidation of CO where chemisorbed capability of gold disturbed on altering mild reduction conditions. Researchers have successfully shown that on tuning the metal surface interface, it is possible to make a stable and successful catalyst that would have many interesting applications. Redox behavior and fairly new materials make them of significant interest. In the esterification and etherification dehydration reaction, cesium containing heteropolyacid found an efficient catalyst. Ytterbium and erbium containing ceria-based materials have a high impact on destroying lungs cancer and used as clinical contrast agents. Ceria based materials have gained significant interest due to numerous applications. Nanoceria has special applications in the prevention of retinal degeneration induced by intracellular peroxides, as a mediator in inflammation of reactive oxygen production and also acting as a protector against ischemic stroke in living animals [7, 17, 24, 33, 45, 55].

Cerium and ceria-based materials as catalyst

The Cerium and Cerium-based materials have been employed as catalyst in exhaust convertors, water gas shift reaction and in hydrocarbon reformation. The reason behind this interesting catalytic phenomenon of cerium and cerium based oxides arises due to excellent redox capability of cerium ion. This redox behavior is only responsible for oxygen storage capacity (OSC) of cerium containing oxides. Based on temperature, the nature of the reactant and employed reaction condition ceria exhibit versatile acid-base properties. It can be absorbed chemically on pyrrole, any proton donor, and carbon dioxide which evidently show the presence of strong Lewis-base sites. The chemisorbed CO2 depends upon the temperature. At high temperature, the amounts of absorbed CO2 decrease while at a low temperature, it increases. Binet and co-workers revealed the acidic site on ceria, by absorption of CO and pyridine, but it had less number of Lewis-acid sites than zirconia or titania [56]. This acid-base or redox surface properties are responsible for its versatility in organic synthesis. In Fig. (1), we have shown ceria application in three parts: activity due to acid-base sites, redox centers, and both acid-base as well as redox sites (Fig. 2) [57-59].

Fig. (2))

Ceria-based heterogeneous catalyst in organic transformation.

Ceria-based material as anticancer

Ceria is widely explored in various fields such as drug delivery, biomedical imaging and therapeutics [59, 60]. Especially nanosized ceria and ceria-based materials exhibit remarkable efficiency against cancer and neurodegenration by offering chronic inflammation and by releasing oxidative stress. Such studies can be sufficient enough for innovative growth in pharmacology and therapeutics. Recent studies on nanostructured ceria and ceria-based material include the study of hollow and porous materials which have gained much attention of researchers. In recent studies, porous materials acting as a good host for drugs, proteins or DNA, protect them from fast reactions occurring in a biological solution. In addition, ceria with hollow shaped morphology is found to be a perfect host for drug delivery [61]. Pristine ceria and CeO2-TiO2 composite with hollow sphere act as a nanocontainer to encapsulate Ag nanoparticles and as a microbicide to kill different kinds of bacteria and fungi [62, 63]. There are only a few reports available on biomedical application of hollow ceria and ceria-based compounds. Though reported literature is in agreement that compounds with hollow morphology will have more preference in improving the biomedical concert of assorted applications; few of them are drug delivery, cancer treatment, enzyme mimetic biochemical catalytic therapy, nitric oxide radical scavenging, hydroxyl radical scavenging and many more [61-63].

Cerium-based material as coatings and pigments

Nowadays, pigment industry is focusing only on designing new smarter pigments which can also be named as nontoxic cool pigments [64]. By reflecting back the solar radiations, cool pigments cause substrate cooling and reduce energy transfer by radiation. In the current scenario, inorganic pigments, those which are biodegradable in nature, are widely researched and explored for their solar as well as NIR properties [65-67]. Wide applicability of yellow and red inorganic pigments is responsible for their high demand in the pigment industry. Bright yellow pigments are highly needed to replace the toxic (Cd and Pb) color pigments available in markets for exterior coatings. The Color Pigment Manufacturers Association, Inc. has shown that the various complex inorganic oxides such as rutile, spinal, hematite and many more are helpful in providing good red or reddish brown contrast. Although these inorganic oxides have limitations that they lack thermal and chemical stablity [68]. Though, there is a high need to develop red/brown pigments with large thermal stability and will be of high technological and environmental importance. There are numerous reports which include newly synthesized ecofriendly red and yellow inorganic pigments with high NIR reflectance [69-73]. Danielson et al. examined the role of a newly prepared cerium-based oxide i.e. Sr2CeO4 as an inorganic color pigment [74]. High thermal and chemical stability of Sr2CeO4 along with unique optical properties is responsible for its wide use in a range of optoelectronic applications [75]. Eu³+-doped Sr2CeO4 is also principally studied as a red phosphor material [76]. Sreena et al. have developed terbium-doped Sr2CeO4 a bright yellow pigment with high NIR reflectance which is considered as a high energy saving product [77].

Ceria-based materials as solid oxide fuel cells (SOFCs)

Since the past few years, solid oxide fuel cells are gaining continued momentum as they pave a